The present invention relates to a colour-stable solution of dimethylaminoacetonitrile in water and to a process for preparing it.
Aminoacetonitriles have been known for many decades (W. Eschweiler, Annalen 1894, 279, 34-44). The industrial production of representatives of this class of compounds is described in, for example, DE-A-2503582. These compounds are valuable starting materials for various fine chemicals. For example, various derivatives of glycine can be produced from these products. Thus, the potassium salt of N,N-dimethylglycine can be obtained from N,N-dimethylaminoacetonitrile by saponification with KOH (DE-A-2503582). Reaction of N,N-dimethylaminoacetonitrile with chlorine gives tetrachloroethylene bisisocyanide dichloride, a starting compound for the fungicide 2-methylimino-3-(4xe2x80x2-chlorophenyl)-4,5-bis(trifluoromethylimino) (DE-A2748964). N,N-Dimethylaminoacetonitrile is also a synthetic building block for xcex1-sinensal, a sesquiterpene aldehyde used as a flavour (GB-A-1467751).
Dimethylaminoacetonitrile (hereinafter referred to as DMAA) is prepared using the amine and liquid hydrocyanic acid together with a formaldehyde source (formalin, paraformaldehyde). The usual procedure is to combine an aqueous formaldehyde solution slowly with liquid hydrocyanic acid and an aqueous dimethylamine solution [1st step of the Strecker amino acid synthesis (H. Beyer, Lehrbuch der Organischen Chemie, 23rd edition, Hirzel Verlag, Stuttgart, 1998, p. 302)]. The reaction can be carried out in a stirred vessel or alternatively in a continuously operated mixing apparatus. 
The reaction of paraformaldehyde with liquid hydrocyanic acid and an aqueous dimethylamine solution also leads to the desired end product. It should be noted here that the use of a solid (paraformaldehyde) in industry usually leads to undesirable and costly additional complication, which is accepted in the synthesis of glycine (Ullmanns Encyclopxc3xa4die der technischen Chemie, Volume 7, 4th edition, 1974, VCH, Weinheim, p. 432).
An aqueous solution of DMAA is obtained after the reaction. Further chemicals which may be present in the end product, usually in low concentration, are the amine used, residual formalin, HCN, the intermediate dimethylaminomethanol, bis(dimethylamino)methane and various stabilizers. The stabilizers are, for example, substances by means of which the aqueous formaldehyde solution is stabilized. In many cases, the purity of the DMAA solution is of critical importance. Thus, the concentrations should be as low as possible, in particular as regards the starting materials and by-products which are harmful to health.
It may also be found that the quality of the DMAA solution deteriorates on storage. Here, the colour of the solution in particular is a feature which worsens during storage. For this reason, usual industrial practice is to process the DMAA solution further immediately or to distil it before storage. The former is often not possible for logistical reasons and the latter is cumbersome and makes the DMAA solution significantly more expensive.
There is therefore a need for an inexpensive process in which a very pure DMAA solution is obtained directly in a storage-stable form.
It has surprisingly been found that a colour-stable aqueous solution of DMAA is obtained when the aqueous formaldehyde solution is stabilized with 2,4-diamino-6-phenyl-1,3,5-triazine(benzoguanamine).
An aqueous formaldehyde solution is customarily stabilized by means of an alcoholxe2x80x94usually methanol. However, if the formaldehyde solution is stabilized with benzoguanamine, gentle reaction of this solution with dimethylamine (preferably in aqueous solution) and liquid hydrocyanic acid gives a DMAA solution which is very pure and storage-stable without further work-up. A DMAA solution prepared according to the invention has a storage stability equivalent to that of a distilled DMAA solution and is considerably superior, particularly in respect of colour stability, to a conventionally prepared solution.
The stabilizer benzoguanamine is particularly advantageously used in a concentration of from 0.05 to 0.7% by weight, based on formaldehyde.
A suitable DMAA solution is obtained when the dimethylamine is reacted with liquid hydrocyanic acid and a formaldehyde source (formalin or paraformaldehyde) in the presence of 2,4-diamino-6-phenyl-1,3,5-triazine and water for from 5 minutes to 5 hours at a temperature of from 0 to 50xc2x0 C. If the formaldehyde source is initially charged and, after addition of the aqueous amine solution a little at a time, dimethylaminomethanol is prepared as an intermediate, the liquid hydrocyanic acid can subsequently be added quite rapidly. However, the temperature should not exceed 500xc2x0 C. in either step; both steps are particularly advantageously carried out at from 25 to 350xc2x0 C. The reaction is also suitable for a continuously operated process. Here too, the reaction temperature should remain below 50xc2x0 C.
The starting substances can be used in a stoichiometric amount or in a slight excess; preference is given to using 0.98-1.02 mol of formaldehyde and 0.95-1.10 mol of dimethylamine per mol of hydrocyanic acid.
The invention thus provides a process for preparing an aqueous DMAA solution from a formaldehyde source, in particular an aqueous formaldehyde solution, dimethylamine and hydrocyanic acid, wherein the reaction mixture contains 2,4-diamino-6-phenyl-1,3,5-triazine(benzoguanamine) in a concentration of from 0.001 to 5% by weight, based on formaldehyde, prior to the reaction and the starting substances are used stoichiometrically or in a ratio of 0.98-1.02 mol of formaldehyde and 0.95-1.10 mol of dimethylamine per mol of hydrocyanic acid.
As formaldehyde source, it is possible to use paraformaldehyde or formaldehyde in gaseous or liquid form or formaldehyde as an aqueous solution. In the latter case, a formaldehyde concentration of from 20 to 50% by weight in water is advantageous; an aqueous solution having a formaldehyde content of from 30 to 40% by weight is particularly useful. The dimethylamine can be used in liquid or gaseous form or advantageously in aqueous solution. In the latter case, the dimethylamine is present in the aqueous solution in a concentration of from 20 to 80% by weight, particularly advantageously from 35 to 65% by weight.
The surprising action of the stabilizer used according to the invention is restricted to an aqueous DMAA solution, since few other amines form water-soluble aminoacetonitriles.
It could be assumed that the unsatisfactory storage stability of the conventional DMAA solution is attributable to the presence of methanol as formalin stabilizer. The presence of methanol can be avoided by the alternative use of paraformaldehyde. This alternative is associated with the problems of solids processing and correspondingly higher costs, which is undesirable. Although a quite clean DMAA solution prepared using paraformaldehyde has a very good colour (which is fully comparable with that of a distilled solution) immediately after the synthesis, it likewise tends to become distinctly darker during storage.
It can thus be stated that a 2,4-diamino-6-phenyl-1,3,5-triazine-containing DMAA solution is significantly more colour stable than a 2,4-diamino-6-phenyl-1,3,5-triazine-free solution prepared using paraformaldehyde. Only when the DMAA solution has been subjected to costly distillation (in which 47% of DMAA together with 53% of water go over as an azeotrope; R. A. Turner, J. Am. Chem. Soc. 1946, 68, 1607-1608), can the presence of 2,4-diamino-6-phenyl-1,3,5-triazine be dispensed with. The synthesis in the presence of 2,4-diamino-6-phenyl-1,3,5-triazine thus gives a colour stability which is similar to that which can be achieved by distillation, but saves this additional and costly process step.
The invention therefore also provides a DMAA solution containing 2,4-diamino-6-phenyl-1,3,5-triazine. The concentration of 2,4-diamino-6-phenyl-1,3,5-triazine in the DMAA solution is from 0.001 to 2.0% by weight, based on the DMAA, preferably from 0.01 to 0.25% by weight.